Near field communication is a wireless communication system operable to effect digital contactless communication between devices at a range of less than 20 cm and usually less than 10 cm and at a carrier frequency of 13.56 MHz. The specifications of methods and devices is mainly published by the NFC Forum (www.nfc-forum.org). Specifications published prior to the filing or priority date of this patent specification are hereby incorporated by reference.
In all NFC communications an initiator (reader) initiates communication by means of an initiation process. The initiation process starts by generating an RF field at 13.56 MHz. An NFC technology specific poll signal is then transmitted by the initiator. The poll response signal sets one or a sequence of timing slots in which to receive a poll response signal and waits for the first timing slot. The NFC listening devices within range, and of corresponding NFC technology, then respond to the poll signal with a poll response signal including the unique identification (UID) of the NFC listening device. The NFC listening device or devices may randomly select timing slots when there is more than one available. The initiation process is complete when the initiator has acquired the UID's of all the listening device in range the initiator is able to command data delivery from each or any specific device, one at a time and in an orderly sequence.
NFC uses only one carrier frequency and so communications between the initiator and listening device must be sequential to avoid interference (collision) between the signals.
NFC communications may be either passive or active. In passive communication the initiator generates the RF field. The listening device load modulates the existing RF field to achieve communication. Passive listening devices may be powered via the RF field and as with RFID devices have been used to tag items or people for many purposes. With active communication both the initiator and listening device generate an RF field. Embodiments herein concern passive communications.
NFC communications may occur between a dedicated device, that is to say devices such as a dedicated reader which is capable of acquiring data from a passive tag. However, smart electronic devices such as mobile (cell) phones, tablets, PDA's and lap top computers, provide an non-exhaustive and growing list of NFC enabled devices. Such devices are adapted in hardware and software to emulate either or both an NFC initiator device and an NFC listening device according to the instant wishes of the user. Embodiments herein apply to both dedicated and NFC enabled devices.
As a result of the proliferation of NFC devices it is increasingly common for an initiator to initiate a response from several listening devices in range. There is therefore a probability of a collision between the responses of the NFC listening devices which will corrupt each device UID or other parts of the response signal. NFC forum has technology specific anti-collision protocols which may allow the resolution of such collisions. For example NFC-A collision resolution is described in detail at the NFC Specification Activities Chapter 9.3.3 FIG. 7, collision resolution for NFC B is described at NFC Specification Activities Chapter 9.3.5 FIG. 8 and collision resolution for NFC-F is described at NFC Specification Activities Chapter 9.3.6 FIG. 9.
Collision resolution activities are either of (i) probabilistic or (ii) deterministic. In the former case the initiator sets a sequence of time slots during which a poll response signal can be received from an NFC listening device. Each activated NFC listening device randomly selects a time slot into which to respond. If and when a clear poll response is received with the NFC UID, the listening device identified by the initiator is commanded into a sleep mode unresponsive to the poll request signal. The process is repeated until no unidentified devices remain active. Because the process is probabilistic it can be prolonged and may fail.
The deterministic method starts by asking for the first number of the tag (a so called “Query Tree” algorithm) until it matches the tags then it continues to ask for additional characters until all tags within the region are found. This method is slow, and introduces a long identification delay but leads to fewer collisions, and has a 100 percent successful identification rate.